Kill Void in strict optional mode

[ddfisher]

This preserves the vast majority of "Function does not return a value" warnings provided by Void.

Fixes #1975.
Fixes #1956.

[ddfisher]

It looks like this fixes roughly 30 of the mypy --strict-optional mypy errors.

[gvanrossum]

I'm feeling lucky today. :-)

[rwbarton]

Yay!

I'm not super enthused about having "mostly equal" types NoneTyp(is_ret_type=False) and NoneTyp(is_ret_type=True), since it becomes unclear in which contexts the difference matters. For example what is join_types(NoneTyp(is_ret_type=False), NoneTyp(is_ret_type=True))? Or does it not really matter? I wonder if you can achieve the same result with purely syntactic checks.

Still very happy to see the Void/NoneTyp distinction gone though.